Car Remotes:

If you repaired a car remote or just changed the battery, you might wonder if the remote really works as a RF transmitter.  Here's an easy way to check basic operation.

Things needed:  Oscilloscope, scope probe, germanium diode (more on the diode later)

Make a loop with the ground wire.  Connect the diode with cathode towards the probe tip.  Set the scope for about 200mV per vertical division and 100msec per horizontal division.  Free run or roll mode are helpful during scope setup.

Put the car remote inside the loop.  Arm the scope and push the button.

A strong burst of pulses is visible.  It's about 3.8V (demodulated) and 180msec wide for the VW remote.

Digital data is visible when zoomed into the later part of the burst.

Similar test for a Subaru remote.  Only 352mV detected in this test setup.  This remote operates at a higher frequency, so the detected signal is smaller.

That Diode:

Multiple diodes were tried.  The only one that worked well was a classic germanium diode, 40-50 years old.  Maybe 1N270 type, but the part was not marked.  Glass germanium parts are still available from surplus and hobby electronics sellers.

A rectifier diode, 1N4004, gave no signal.  It is too big and slow.

A silicon switching diode, 1N4148, gave a very weak signal.  Maybe the Vfwd is too high.

A signal Schottky might work, but this wasn't available from the parts bin.

The silicon diode might have worked better with some DC bias and coupling, but then it's not a simple test setup.

Frequency:

The car remotes operate at 315 or 433.9 MHz.  This is too high for direct measurement by an ordinary oscilloscope.  The diode acts as an AM demodulator to detect the pulse modulated signal.

The loop and diode worked best at 315MHz.  It worked OK at 433.9MHz, but the signal level was lower.  This might be related to the loop.  The loop has a small inductance and this will resonate with the capacitance at the probe tip.  433MHz is probably farther above that resonant frequency, so coupling is poor, even at close range.

Frequency Lookup:

It's possible to look up the frequency of a car remote from the FCC ID.